Refactor control module improvements

[pnorton-usgs]

Minor refactoring and bug fixes across the control module files.

Changes

• Control.py: Use context manager (with open) in write() for safe file handling
• Control.py: Replace assert False with raise NotImplementedError in _read()
• ControlFile.py: Fix filename property return type to Path (setter already converts)
• ControlVariable.py: Simplify _value_meaning_test - remove nested try/except and regex in favor of direct lookups; remove unused re import

Testing

All 54 control module tests pass.